Biosensors are electronic devices which produce electronic signals as the result of biological interactions. Basically, a biosensor includes a biological receptor linked to an electronic transducer in such a way that biochemical activity is converted into electrical activity. The electronic component of the biosensor measures voltage (potentiometric), current (amperometric), light, sound, temperature, or mass (piezoelectric). Lowe, C. R., Biosensors 1:3-16(1985). The development of chemical microsensors has fostered the technology necessary for biosensors. For example, an interdigitated gold electrode with a semiconductor film coating has been utilized in measuring concentrations of organic and inorganic vapors. Wohltjen, J., Analytical Chemistry 56:87-103 (1984).
Two types of biosensors are known: enzyme-based or metabolic biosensors and binding or bioaffinity biosensors. Enzymatic or metabolic biosensors use enzymatic or metabolic processes to detect the product of the reaction which occurs between the incoming agent (substrate) and the immobilized receiver (e.g., an enzyme). Enzyme-based biosensors are best exemplified by enzyme electrodes, which are devices which utilize standard electrodes able to detect dissolved gases, such as oxygen or chemicals, such as urea, electronically. When enzymes attached to the electrodes catalyze a reaction, a gas or chemical is produced. This chemical or gas is detected by a specific electrode, for example, an oxygen or ammonia electrode. Perhaps the best known examples of enzyme electrodes are those which contain glucose oxidase or urease. They can be used to measure, respectively, glucose or urea, as well as to detect end products of multi-enzyme systems (for detection of other substrates). Such enzyme electrodes are well-defined and many are commercially available. Vadgana, P., Journal of Medical Engineering Technoloqy, 5:293-298 (1981); Solsky, R.6., CRC Critical Review of Analytical Chemistry, 14:1-52 (1983).
Bioaffinity sensors rely on biological binding events for detection of substances of interest. Taylor, R. F., The World Biotech Report 1986, Vol. 2, pp.7-18 (1986). The binding of the environmental substance (ligand) to the immobilized receptor produces a detectable change in the shape or conformation of the receptor and this produces an output signal. Detection of this change can utilize one of a number of methodologies, including optical (interference, refractive index, fluorescence, etc.), mechanical (mass or density) or temperature changes.
Until the present time, only antibodies or antigens have been used successfully for bioaffinity sensors. For example, Wasserman antibody in blood has been detected through the use of a membrane containing immobilized antigen. Aizawa, M., et al., Journal of Membrane Science 2:125-132 (1977). Upon interaction of antibody with antigen, a millivolt (mV) change in potential occurs across the membrane; the change is proportional to concentration of antibody present in the blood sample. Antibody-antigen binding is also used in a variety of optically-based biosensors. Place, J. F., et al., Biosensors 1:321-353 (1985). The basic action mechanism in this type of biosensor has not been defined, but appears to be a change in conformation of the immobilized receptor and/or a physical change in the immobilization media (e.g., weight, thickness, light absorbancy, etc.). These changes are detected and amplified electronically using appropriate transducer technology.
There have been attempts to develop other types of binding or bioaffinity sensors. For example, Yager describes a biosensor consisting of a polymerizable lipid bilayer which contains an active membrane protein (e.g., the acetylcholine receptor) and which separates two electrolyte-filled compartments. Synthetic phospholipids in the bilayer membranes are used as stabilizers, and the membrane proteins are incorporated through use of a modification of known methods. Changes in current across the receptor-containing membrane are described as occuring when cholinergic agents are bound and measured using a known (electrode patch) technique. Yager P., U.S. Statutory Invention Registration H201 (Published Jan. 6, 1987).
Others have described efforts to immobilize enzymes and other "bioactive" materials onto glass (U.S. Pat. No. 4,357,142) and other surfaces, such as those containing acrylate copolymer (U.S. Pat. No. 4,352,884) or an acrylonitrile polymer (U.S. Pat. No. 4,371,612). See also U.S. Pat. No. 4,307,195; U.S. Pat. No. 4,456,522; U.S. Pat. No. 4,415,666; U.S. Pat. No. 4,418,148; and U.S. Pat. No. 4,484,987.
Although antibody- or antigen- based biosensors are useful in detecting ligands in samples, they have limitations, such as over-selectivity and near irreversible binding, which make it impossible to use them in many instances. Biosensors which are binding sensors or bioaffinity sensors, and which make use of a receptor other than an antibody or an antigen would be very valuable and find use in many contexts in which presently-available binding sensors cannot be used.